But as good as fast-moving roads might be for the individual driver, they are not the best for the most drivers. As data gleaned from in-pavement “loop detectors” on Washington state highways showed, those highways were able to achieve “maximum throughput” — pushing the most cars through one segment of road in a given time — at speeds that were roughly 80 percent of the posted speed limit of 60 mph. Why? At higher speeds, drivers need to allow more “headway” between vehicles, meaning more space is required per vehicle. And faster-moving traffic tends to break down more quickly, with more severe “shock waves”; it takes a lot longer to recover from a traffic jam than to get into one. I have been told, anecdotally, by traffic engineers that the left-hand “passing lane” can become congested first. (I’ll leave it to you to decide if karmic justice is at work there.)This goes along with an idea I had about having a fleet of driverless cars using real time data to selectively slow traffic in some or all lanes for the purpose of reducing the accordion effect. If nothing else it would be an interesting project to model.
In a “speed harmonization” experiment on Colorado’s I-70, the state highway patrol was able to improve traffic flow by enforcing, via phalanxes of patrol cars, 55 mph speeds — thus preventing the instability caused by people driving fast into packs of congested, slower-moving vehicles.
Thanks to Mike for the link.